This invention relates to couplers for optical fibers used in communication cables. More particularly, this invention relates to fiber optic signal splitters for single-mode-type optical fibers.
Signal splitters are among the most complex and expensive components of a fiber optic system. Both multiple-mode and single-mode fiber systems have a need for efficient splitters. A multiple-mode fiber supports light propagation in its core in a transverse modal pattern consisting of two or more modes as defined by the boundaries of the waveguide formed by the fiber core. Single-mode fibers have a much thinner core and substantially thicker cladding about the core and thereby are constrained to support only the single fundamental propagation mode. Due to the relatively small diameter of the core or waveguide portion of a single mode-optical fiber within a cladding, as compared to the multiple-mode optical fiber, it has heretofore been extremely difficult to construct efficient splitters using single-mode fibers.
A typical prior discrete component splitter for single-mode fiber employed a fused bundle in which all leads except one fiber lead on one side of the fused junction are simply clipped off or ignored. This leaves many unterminated ends. As a consequence, such a splitter is highly susceptible to mechanical failure, spurious input and signal loss.
Other splitters are limited in number of branches due to constraints on cross-sectional configuration, often requiring precise placement and spacing in cross section, or constraints on available space. Integrated optical elements, for example waveguides on a chip, are restricted by available pin-out and fiber-coupling space on the chip. Hence, they are not practical for applications requiring a large number of outputs.
A number of patents were uncovered in a review of the state of the art related to non-integrated optical couplers. Much of the art uncovered dealt with multiple-mode structures and are therefore not directly relevant to single mode structures with the different core geometry. The patents uncovered are discussed briefly hereinbelow.
U.S. Pat. No. 5,138,677 relates to multi-mode fiber, a different technology due to the substantially different ratio of cladding to fiber core. This patent illustrates a multiple fiber bundle in an N to 1 configuration wherein an etching process is employed to remove substantially all of the cladding.
U.S. Pat. No. 5,101,462 discloses a two-fiber coupler. The structure and purpose would not lend itself to a 1 by N splitter. In the construction, a preheat step is required.
U.S. Pat. No. 4,362,357 discloses a specific multimode fiber structure.
U.S. Pat. No. 5,175,782 discloses a structure with a controlled geometry which requires the use of an outside layer. The structure is limited to no more than an 8 to 1 split.
U.S. Pat. No. 5,216,731 discloses a machine for building optical fiber couplers which might be adapted for use in construction of a splitter according to the invention.
U.S. Pat. No. 5,140,655 discloses a bidirectional optical star, which is one potential application of the present invention.
U.S. Pat. No. 5,170,451 discloses another potential application of the present invention.
U.S. Pat. No. 5,171,345 discloses a mechanism wherein two fibers are fused. The invention therein would exhibit uneven fiber heating and uneven drawing of fiber.
U.S. Pat. No. 5,175,779 discloses a mechanism whereby a fiber bundle can be fused and drawn using a collar structure.
U.S. Pat. No. 5,190,364 discloses the heating of a fiber in a large chamber, and the covering of an entire bundle with a dielectric layer.
U.S. Pat. No. 4,522,461 discloses an optical multiplexer with a graded index rod lens.
U.S. Pat. No. 5,121,452 discloses a fiber optic power splitter comprising a geometry-specific twisted bundle of fibers having a required center fiber.
In a poster preview session of the OFC/IOOC conference on Oct. 13, 1993, Himeno et al presented a description of an optical-fiber-couple fabrication technique using multicore fibers. A description was contained in the accompanying technical digest (Himeno et al., "Novel optical-fiber-couple fabrication technique using multicore fibers," WG1, OFC/IOOC '93 Technical Digest). Therein a technique was described wherein a bundled fiber set was arc-fusion-spliced to a tapered elongated multicore fiber of the same number of fibers and same diameter but with evidently no attempted alignment between multiple bundled fibers and a single elongated multicore fiber. The technique does not disclose how to fabricate an individual mode star coupler for a single single-mode fiber to a bundle of single-mode fibers, and it teaches that alignment of fibers is not needed.
In the preparation of an optical fiber to construct a coupler, particularly a multiple mode coupler, it is known to strip off the outside plastic covering, called a buffer, and to etch off substantially all of the quartz fiber cladding to yield a thinned fiber consisting of a high refractive index quartz core. Multiple-mode fibers may then be fused in parallel to one another.
What is needed is an efficient, low-loss and low-cost splitter for single-mode fiber which is relatively easy to construct and which can serve to multiplex a single output signal of a single fiber into an arbitrarily large number of output fibers.